Solenoid operated directional valves with detent means are known in the art. These valves have detent means which make direct contact with the spool for holding the spool in a selected position. The spool is moved by a solenoid pin that has been shifted axially in response to the energization of the associated solenoid. To provide holding engagement of the spool by the detent means, it has been the general practice to provide the spool with a cylindrical surface, with or without special grooves, notches, or the like, for engagement by the detent means, and the detent means usually is located in a flow passageway adjacent to the end of the spool for making contact with these surfaces. The known detent means may make point contact with the spools, such as occurs in one structure wherein spring actuated ball-detents are used, or the detent means may make line contact with the spool as is done in a second known prior art structure in which especially shaped spring clips are employed. In a third type of detent means, essentially a line contact is realized wherein an elastomeric ring engages the cylindrical end surface of the spool to hold the latter.
The three types of detent means set forth above are commonly used, but they have numerous inadequacies. Ideally, the detent means should have the following properties: (a) the disengagement force for disengaging the detent means should be substantially constant; (b) the detent means should have an infinite life when compared to the life of the valve; (c) the detent means should not apply a side load on the spool; and (d) the parts of the detent means should be confined so that loose parts are avoided.
In general, the detent means that make point contact with the spool are deficient to some extent in respect to all of the requirements enumerated above. Other detent means, such as spring clips, which make line contact with the spool must be tailored to get uniform characteristics in disengagement, thus increasing production costs. They also have a finite life, and they apply side loads against the spool. Those detent means that employ an elastomeric ring for holding the spool lack the sensitivity required, because the elastomeric rings are temperature sensitive, and the properties of the elastomeric rings during initial manufacture may differ from one batch to the next. They also are subject to wear so as to vary their disengagement characteristics as well as to limit their lives.
With respect to all of the prior art detent means, it is the general practice to mount them so that they can make direct engagement with the spool, and to accomplish this mode of operation, the detent means are located in flow passageways into which the ends of the spools can be shifted. When in these locations the detent means partially obstruct the flow of the hydraulic fluid through these passageways and thereby create undesirable pressure drops.